With theese parts I have rebuilt my K8200 to double Z-axis.
You will need the following
1 X original T5 belt (I have replaced my T5 with GT2 belt) joined with superglue.
3 X 608 bearings 22x8x7
2 X Trapetzeoid spindles TR8X1,5 or whatever
1 X Flexible coupling 5mm/8mm
2X M8 Nylock nuts
Bolts, nuts etc.
You need to adjuste the values for steps / mm according to your selection of trapetzeoid spindle.
See video for it in action.
Update: Credits to Stimpy for his original design!